The present invention is related to the art of growing silicon films on ceramic substrates by bringing the substrate in contact with molten silicon. The present invention is broadly related to prior art U.S. Pat. Nos. 4,112,135, 4,128,680 and 4,137,355 by Joseph D. Heaps and Obert N. Tufte, all assigned to the same assignee as the present invention, where there is described apparatus and method of coating ceramic bodies or sheets with molten silicon to prepare large area, thin sheets of large grain polycrystalline silicon on inexpensive ceramic substrate for use in solar cell panels and the like. This is referred to as silicon-on-ceramic or supported growth. In that method the side of the ceramic sheet or the area to be coated with silicon is first carbon coated, i.e. carbonized. It is taught in the cited references that when a ceramic (which normally is not wet by molten silicon) is first carbonized on a surface to be silicon coated, the carbonized surface will then be wet by molten silicon, and by contacting the carbonized ceramic substrate with molten silicon, a silicon coating will be formed thereon.
The present invention is directed to an improvement over the prior art silicon-on-ceramic technique in providing increased growth speed, in providing rejection of unwanted impurities, in providing a silicon layer with a natural impurity segregation which gives a graded impurity level through its thickness, and in providing a built-in back surface field in the silicon layer that will enhance solar cell performance. The improved growth technique lies in what may be described or classified as an asymmetric mode of growth of the silicon-on-ceramic in contrast with the symmetrical mode of growth occuring in the process of the prior art patents cited above. This growth technique is classified as an asymmetric mode of growth of silicon on the substrate, because the substrate must remain hot (i.e. at a higher temperature than the solidification temperature of silicon) in the area of the substrate where the silicon layer growth is taking place so that the liquid-solid interface is nearly parallel to the substrate, but inclined at a reentrant angle, so that the leading edge of the crystallization front (first to solidify) is away from the substrate.